Chroma-key circuits are well known in the broadcast industry. By the use of such circuits it is possible to transmit composite pictures which alternately use the outputs from two separate cameras. For example, a street scene may be transmitted wherein the face of an announcer may be compositely placed in one corner to form the composite picture. Also, the composite presentation may depict an action scene appearing as the background of the picture, with the announcer standing the midst of the action scene.
Chroma-key techniques are used to affect such composite broadcast signals, for example, by placing the announcer in front of a single-color backdrop, and by sensing changes in the output from the announcer's camera to develop an output signal for switching between the announcer's camera and the action camera. For example, whenever the single-color backdrop is picked up during the sweep of the announcer's camera, the synchronized sweep of the action camera output is transmitted; while the output from the announcer's camera is transmitted whenever there is an absence of the backdrop color during the sweep of the announcer's camera.
A conventional method of chroma-keying utilized by the broadcast industry is to sense the ratio between the red, green and blue output signals from the announcer's camera, and when these ratios correspond to a predetermined value the chroma-key circuit operates to key the action camera. An absence of the predetermined ratio will cause the circuit to key the announcer's camera. The ratio should be a linear relationship, and the circuitry necessary to determine the linear relationships between the various colors and to generate the keying signal used to switch between the two cameras is quite complex and therefore expensive. Furthermore, cameras produced by some manufacturers do not provide separate red, green and blue output signals wherefore the conventional chroma-key circuit described above cannot be used without additional demodulating circuits. One patented example of circuitry which processes the three individual color signals is described in U.S. Pat. No. 3,678,182.
Accordingly, an object of the present invention is to provide a less complex chroma-key circuit capable of utilizing the standard composite NTSC signal which is generated by most known color cameras in the United States.